The present invention relates to the production of double-face corrugated paperboard webs formed by laminating flat paper webs to the opposite sides of a corrugated medium and, more particularly, to a method and apparatus for controlling the application of heat to the webs as a function of production factors to aid in the control of warping of the corrugated paperboard.
Corrugated paperboard is manufactured at high production rates on corrugator machines which are well known in the paper industry. A typical corrugator machine joins a flat web usually referred to as a single-face liner to a corrugated medium to form a single-face web. The single-face web is then joined to a second flat web typically referred to as a double-face liner by gluing the liner to the opposite side of the corrugated medium to form a double-face web. The double-face web forms the corrugated paperboard and is typically slit, scored and cut into predetermined lengths to form corrugated paperboard blanks for the production of paperboard containers.
One particular difficulty that has plagued the corrugated paperboard industry for years is that the finished blanks tend to be warped in one or more directions, making it difficult or impossible to form them into containers. This tendency has been attributed at various times to different production factors such as residual stresses, moisture variations, adhesive quantity, induced tension and heat transfer characteristics. Many corrective methods in systems have been used with limited degrees of success as is discussed in U.S. Patent Application Ser. No. 520,687 by Thayer et al, assigned to the assignee of the present invention and hereby incorporated herein by reference.
One factor influencing warp is the amount of heat applied to the various webs or lamina before they are joined as well as heat applied to the single-face web and double-face liner before they are joined. The application of moisture and heat is normally referred to as preconditioning and may result in dimensional changes in the lamina.
In one known system, preheaters are used prior to the single-facing operation and the double-facing operation to control the application of heat to the single-face liner, the single-face web and the double-face liner. The preheater includes a preheater roll into which steam is introduced to heat the roll to a desired temperature. The paper passes over the preheater roll in contact with a selectable circumferential area of the roll. In order to vary the contact area between the paper and the roll, a wrap roll is positioned relative to the preheater roll to vary the angular position at which the paper first contacts the preheater roll and thus the amount of "wrap" of the paper around the preheater roll.
The amount of heat applied to the paper (i.e., the exposure time of the paper to the preheater roll) is thus controlled by controlling the angular position of the wrap roll relative to the preheater roll. In known systems such as that shown in Japanese Pat. No. 49-37994, the exposure or contact time between the paper and the preheater roll is maintained at a constant value through the use of a closed loop servo system with position feedback and speed related control. More specifically, the speed of the paper through the corrugating machine is sensed together with the angular position of the wrap roll. A comparator circuit compares the speed value with the angular position value and a motor drives the wrap roll in the proper direction until a null is obtained between the compared values.
In such an analog closed-loop system, the angular position of the wrap roll must be sensed and provided in the form of a feedback signal in order to correlate position with speed. Additionally, closed-loop analog control is not compatible with digital control systems and, particularly, with open-loop computer control.
It is accordingly an object of the present invention to provide a novel open-loop, digitally operable method and apparatus for controlling the application of heat to webs in preconditioning sections of corrugating machinery as a function of production factors.
It is another object of the present invention to provide a novel wrap roll control circuit and method which is compatible with digital control techniques.
It is yet another object of the present invention to provide a novel open-loop wrap roll control circuit and method which controls the position of the wrap roll of a preheater in accordance with production factors without the necessity for sensing wrap roll position.
These and other objects and advantages of the present invention are provided in accordance with the present invention as will become apparent to one skilled in the art to which the invention pertains from the following detailed description when read in conjunction with the appended drawings.